The Cardiac Cycle

Question 1

Learning outcomes

Answer 1

• To describe the events of the normal cardiac cycle
• To show how it is controlled by the cells specialised for rhythm generation and conduction that were described in the last lecture
• To fully delineate the pressure and volume changes seen in the heart chambers, conducting arteries and
great veins during the cycle.
• To describe the normal and abnormal sounds that may be detected when listening to the heart

Question 2

Atrial systole

Answer 2

• Atrial systole
• Atrial contraction primes the main pump. However 70% of ventricular filling is passive.
• Contraction of muscle around great veins minimises backflow along with inertia of blood
• a wave of atrial pressure and JVP trace
• 0 - 0.1 s into cycle

Question 3

Ventricular systole

Answer 3

• Ventricular systole
• Isovolumetric contraction; ventricles contract against closed valves until Pvent > Part.
Builds up pressure head
• Aortic and pulmonary valves burst open, giving the ventricular ejection phase.
• C wave of Atrial and JV (jugular venous) pressures
• 0.1-0.4s into cycle

Question 4

Early and late diastole

Answer 4

• Early diastole
• Protodiastole, ventricles stop contracting, blood still flows out through the momentum imparted by ventricular contraction.
• Aortic and pulmonary valves close and we get isovolumetric relaxation until pressure falls to below atrial pressure
• V wave on JVP and atrial pressure trace from blood
flowing into atria
• Then AV valves open
• 0.4 – 0.6 s into cycle

• Late diastole
• AV valves open, blood flows in to the atria
and ventricles passively
• 0.6 – 0.8 sec into cycle

Question 5

Arrythmias- clinical relevance

Answer 5

• Normal sinus rhythm between 60 -200 BPM, sinus arrythmia present, particularly in fit people
* Normal rhythm, atria, delay, ventricles*
• Sick sinus syndrome
• Heart block, 3rd degree, second degree, first degree
• Ectopic foci, where part of the heart not normally given to firing signals does. Causes extrasystole Arrythmogenic and potentially lethal
• Re-entry phenomena, also arrythmogenic and potentially lethal
• Circus movements, often with re-entry or after infarct
• All of the above can cause serious arrythmias such as atrial tachycardia, paroxysmal ventricular tachycardia and atrial and ventricular fibrillation
• See also the section at the end of chapter 28 in Ganong on arrythmias for lots more detail.